1. Field of the Invention
The present invention generally relates to electronic video displays, and more particularly to a method and apparatus for calibrating the video output from a computer to optimize color contrast in a liquid crystal display panel.
2. Description of the Prior Art
Liquid crystal display (LCD) panels are known in the art and, in recent years, have increasingly been used in combination with overhead projectors (OHP's) to allow visual presentation of electronically stored graphic images. An exemplary LCD/OHP setup is shown in U.S. Pat. No. 4,846,694. In this conventional configuration, a controller (typically a personal computer) is used to create and store the graphic images. The LCD panel, which is placed on the stage area of the OHP, is electrically connected to the primary or auxiliary video output port of the computer.
The present invention relates to the use of LCD panels to display color (or gray-scale) images. Prior art devices have already overcome several problems in the presentation of color images. The first of these problems was the complete lack of an LCD panel which was even capable of display "true" colors, i.e., combinations of primary colors, either additive or subtractive. For example, the LCD panel disclosed in U.S. Pat. No. 4,944,578 is a "pseudocolor" panel, capable of displaying only certain color combinations such as yellow and blue, based on the birefringent characteristics of the liquid crystal medium. The control electronics for that panel map primary colors from the computer video output to selected colors available in the panel; the color gamut may be varied by changing the operating voltage of the panel, which affects the birefringent response of the panel.
Later LCD panels provided true color by either using primary color triads, similar to the pixel triads in conventional television screens (see, e.g., U.S. Pat. No. 4,791,415), or by providing a three-layer stack of LCD panels, each panel providing one of the three primary colors (see, e.g., U.S. Pat. No. 4,917,465). Other refinements in color LCD technology include: automatic adjustment of color depending upon ambient lighting conditions and (with respect to the spectral luminous efficiency of human eyes) the particular color being used (see U.S. Pat. No. 4,752,771); improved color convergence using synchronous adjustment of the transmission of primary color light gates (see U.S. Pat. No. 4,907,862); and manual adjustment of color contrast to overcome inherent transmittance responses (for each primary color) of the liquid crystal material (see U.S. Pat. No. 4,942,458).
One remaining problem which has not been adequately addressed by the prior art, and which is fundamental to proper color display, concerns the variable manner in which the output video signal is generated by different types of computers. This problem arises when the output is in analog form (the format used by a visual graphics array (VGA), as well as in SuperVGA, and XGA), although it does not occur when output is in digital form (the format used by a color graphics adaptor (CGA) and an enhanced graphics adaptor (EGA)).
In the analog system, there are typically five analog outputs: two clock signals (horizontal and vertical synch), and three analog signals, one for each of the primary colors, viz., red (R), green (G), and blue (B). The industry standard for this type of output (RS-343A) requires that the maximum signal level for a given color (corresponding to full saturation of that color) should be 0.7.+-.0.01 volts peak-to-peak. This analog value is then converted, by an analog-to-digital converter within the LCD panel, to a digital value. For example, in a 512 color display format, such systems typically use a three bit field (for each primary color) to define eight different shades. In other words, a converted, digital value of 000 corresponds to the minimum transmittance (i.e., no transmittance) of the primary color, while a value of 111 corresponds to the maximum color saturation.
Yet, in actuality, the output analog values vary considerably depending upon the particular computer being used to generate the output signals. Due to component tolerances, temperature variations, and product aging, some computers output a maximum signal level of only 0.5 volts, while others output a maximum level of 0.8 volts, while some computers output signal strengths varying within this range. In the former case (underdriving), the converted digital value will be less than the maximum, e.g., in a 512 color scheme, the maximum signal level of 0.5 volts might correspond to a digital value of 110 or 101, instead of 111. In such a case, an LCD panel will fail to provide full color intensity, resulting in poorer contrast and resolution. In the latter case (overdriving), the analog signal corresponding to maximum intensity will be converted to the proper digital value (e.g., the 0.8 volt signal will convert to a digital value of 111); other problems, however, arise in such cases of overdriving. There will be less differentiation between sequential color shades since the next level down in intensity might still result in a digital value of 111. In other words, a light gray shade will appear identical to the full white color on the LCD panel. Moreover, overdriving can affect the proper display of low intensity signals, e.g., a screen which should appear totally black might instead be dark gray.
Prior art LCD panels have incorporated manual adjustment controls, functioning much like the tint adjustment on conventional television sets, which allow the user to compensate for the foregoing effect. This is, nevertheless, a tedious manual operation, and it must be repeated each time the panel is connected to a different computer. Manual adjustment is also very subjective, and presenters will often differ on the settings required for optimum color contrast. Moreover, during a long presentation, changes in temperature (affecting both the computer and the LCD components) may cause the color quality to deteriorate, requiring further adjustment. Finally, several prior art LCD panels providing such manual adjustment allow only a single adjustment for all colors, i.e., they do not provide separate adjustment control for each of the primary colors. It would, therefore, be desirable and advantageous to devise a method and apparatus for simplifying calibration of color signals which is automatic and independent of the particular computer which is generating the video output signals.